Truncation is the crudest fix for a full window: cross out the earliest turns until the new message has room. It is cheap and it always works, which is why agents do it, and it costs the beginning of the conversation, which is usually the part that said what the conversation was for. Turns go whole. You cannot drop half a turn, so the count always rounds up and a little more is thrown away than strictly had to be.
Paid members: the worksheet and its printable PDF are below ↓